Patent Document 1 discloses a drive system of a synchronous motor. The drive system includes a three-phase synchronous motor, an inverter, and a controller. The inverter is connected to the three-phase synchronous motor and includes a plurality of switching elements. The controller controls energization of the inverter by pulse width modulation operation according to six energization modes each for selecting two phases to be energized from among the three-phase coils of the three-phase synchronous motor. The drive system sequentially switches between the energization modes in accordance with the comparison result between a reference voltage and the detected value of the terminal potential (induced voltage or transformer electromotive force) of a non-energized phase.